Method of manufacturing an antiblooming image sensor device

ABSTRACT

An image sensor device comprising a semiconductor substrate having a number of surface-adjoining channel regions. The channel regions are separated from each other by surface-adjoining channel separation zones. The channel regions further adjoin an underlying semiconductor zone extending substantially parallel to the surface. The channel regions have doping concentrations which exceed that of the semiconductor zone. The semiconductor zone has a dopant concentration which exceeds the dopant concentration of the semiconductor substrate. The semiconductor zone has a varying thickness which has minima at the areas of the centers of the channel regions. In such an image sensor device, a potential distribution can be obtained which strongly suppresses blooming realized at right angles to the surface that the occurrence of blooming is strongly suppressed. The invention also relates to a method of manufacturing this image sensor device.

This is a division of application Ser. No. 671,154, filed Nov. 13, 1984,now U.S. Pat. No. 4,654,682.

BACKGROUND OF THE INVENTION

The invention relates to an image sensor device. The image sensor devicecomprises a semiconductor substrate of a first conductivity type havinga surface. A number of channel regions of the first conductivity typeare formed at the surface of the substrate. The channels extend alongthe surface transverse to a system of electrodes present on the surface.During operation of the image sensor device, charge is collected andtransported in the channel regions. The channel regions therefore areseparated from each other by channel separation zones of a secondopposite to the first conductivity type. The channel separation zonesare also formed at the substrate surface. Beneath the channel regions isa semiconductor zone of the second conductivity type extendingsubstantially parallel to the substrate surface.

In such an image sensor device, during operation voltages are applied tothe electrodes to form a pattern of potential wells separated bypotential barriers in the channel regions. For a given integration time,charge produced in the semiconductor material by incident radiation iscollected in these potential wells. Thus, a charge image correspondingto a radiation image is formed. After the integration time, clockvoltages are applied to the electrodes, to transport the charge packetsthrough the channel regions, to a storage register. Such a method iscalled frame transfer or field transfer. Subsequently, the charge isfurther processed during the next integration period to produce atelevision signal.

By the application of suitable voltages between the electrodes, thesemiconductor zone and the substrate, a potential barrier can beproduced in the semiconductor zone. Thus, charge which, viewed from thesurface, is produced above this potential barrier in the semiconductormaterial will contribute to the formation of the charge image. Chargeproduced beneath this potential barrier will not contribute to theformation of the charge image. Since long wavelength radiation canpenetrate more deeply into the semiconductor material than shortwavelength radiation, the spectral sensitivity of the image sensordevice is determined by the position of the potential barrier.

British patent application No. 2,054,961 discloses an image sensordevice in which the semiconductor zone and the channel region havedoping concentrations which do not exceed that of the substrate. As aresult, charge collected in the channel regions can influence thepotential variation between the surface and the semiconductor substrateso that the potential barrier, which was initially present at the areaof the semiconductor zone, disappears when a given quantity of charge isexceeded during the integration period. When during the integrationperiod, high intensity irradiation produces excessive charge, the excesscharge can flow away to the semiconductor substrate. Thus, this excesscharge will not be spread over a large number of adjacent potentialwells present in the channel regions during the integration period. Thisspreading of charge, often designated as "blooming", can give rise tovery disturbing lines in a television picture which is formed fromsignals produced with such an image sensor device.

The known image sensor device comprises a semiconductor substrate onwhich two semiconductor layers are disposed. The upper layer comprisesthe channel regions. Both layers have doping concentrations which do notexceed that of the semiconductor substrate. Such a construction cannotbe obtained by diffusion of impurities in semiconductor materials. Toform layers by diffusion of impurities into a semiconductor body of oneconductivity type, a zone of the other conductivity type can be formedonly by providing a dopant concentration which exceeds that of thesemiconductor body. In order to be a able to manufacture the known imagesensor device, first a layer of the second conductivity type and then alayer of the first conductivity type will have to be grown epitaxiallyonto a semiconductor substrate. In this multi layer structure, channelseparation zones extending into the lower of the two layers can beformed by means of diffusion of impurities in both layers.

Another disadvantage of the known image sensor device is that thechannel regions have widths which are determined during the manufactureof the channel separation zones. The separation zones will have minimumwidths which are equal to the minimum dimensions of the windows requiredfor the diffusion, plus two times the distance over which lateraldiffusion takes place. This diffusion distance is larger in the knownimage sensor device than the thickness of the layer in which the channelregions are formed. Starting from a desired center to center distancebetween adjacent channel regions, the desired center to center distanceminus the width of the required diffusion window and well over twice thethickness of the channel regions is then left for the width of thechannel regions. In practice, the desired center to center distance is,for example, 10 μm, the width of the window is 4 μm and the thickness ofthe channel regions is 1 μm. The width of the channel regions is thenonly about 3 μm.

The known image sensor device consequently has comparatively narrowchannel regions and comparatively wide channel separation zones. This isundesirable because the quantity of charge that can be collected andtransported per unit surface area is comparatively small, and also theimage sensor device consequently has a comparatively low sensitivity.Charge produced in the channel separation zones flows away to thesemiconductor substrate and does not contribute to the formation of thetelevision signal.

SUMMARY OF THE INVENTION

It is an object of the invention to provide an image sensor device inwhich charge collected in the channel regions can influence thepotential variation between the surface and the semiconductor substratein such a manner that excess charge can flow away to the semiconductorsubstrate.

It is another object of the invention to provide an image sensor devicewhich has comparatively wide channel regions and comparatively narrowchannel separation zones.

In the image sensor device according to the invention the channelregions have dopant concentrations which exceed that of thesemiconductor zone. The dopant concentration of the semiconductor zone,in turn, exceeds the dopant concentration of the semiconductorsubstrate. The semiconductor zone has a thickness which varies in adirection perpendicular to the channel regions. The thickness of thesemiconductor zone has minima at the centers of the channel regions.

The image sensor device according to the invention can be manufacturedin a simple manner by diffusing impurities into a semiconductorsubstrate of the first conductivity type. Zones of the secondconductivity type can be provided by diffusing impurities through afirst mask with windows at fixed relative center-to-center distances.Zones of the first conductivity type forming the channel regions can beprovided by diffusing impurities through a second mask with windows atthe same relative center-to-center distances. The second mask is thenarranged so that the channel regions are formed halfway between thezones of the second conductivity type. The channel regions thus formedare then separated by surface-adjoining zones of the second conductivitytype (the channel separation zones). The channel regions further adjoina zone of the second conductivity type extending substantially parallelto the surface. This zone has a thickness, which varies in a directionperpendicular to the channel regions and has minima at the centers ofthe channel regions. The channel regions have dopant concentrationswhich exceed that of the underlying semiconductor zone, while the latterin turn has a dopant concentration which exceeds that of thesemiconductor substrate. As a result, effective blooming suppression ispossible.

In the image sensor device according to the invention, the widths of thechannel regions are equal to the widths of the windows in the secondmask plus the distance over which lateral diffusion takes place. Thisdistance is substantially equal to the thickness of the channel regions.In this case, a limiting factor is the minimum separation which has tobe maintained between two windows. With a desired center-to-centerdistance between the channel regions, the maximum widths of thesechannel regions are equal to this center-to-center separation minus theminimum window distance and plus twice the thickness of the channelregions. In a practical embodiment, the center-to-center distance is 10μm, the minimum window width is 4 μm and the thickness of the channelregions is 1 μm. The widths of the channel regions are then at least 8μm. As compared with the known image sensor device, the image sensordevice according to the invention has comparatively wide channel regionsand comparatively narrow channel separation zones.

The inventor further relates to a method of manufacturing an imagesensor device. In this method zones of the second conductivity type areprovided in a semiconductor substrate of the first conductivity type bydiffusing impurities through a first mask with windows at fixed relativecenter-to-center distances. Zones of the first conductivity type formingthe channel regions are provided through a second mask with windows atthe same relative center-to-center distances. The second mask isarranged so that the channel regions are formed halfway between thezones of the second conductivity type.

Thus, by simple diffusion techniques, an image sensor device is obtainedin which the channel regions are separated by surface-adjoiningsemiconductor zones of the second conductivity type (channel separationzones). The channel regions further adjoin an underlying semiconductorzone of the second conductivity type. The channel regions have dopantconcentrations which exceed that of the semiconductor zone, which inturn has a dopant concentration which exceeds that of the semiconductorsubstrate. The semiconductor zone has a thickness which varies in adirection perpendicular to the channel regions. The thickness has minimaat the centers of the channel regions. The image sensor thus formed canbe used in a manner such that blooming is strongly suppressed.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a plan view of embodiment of the image sensor device accordingto the invention.

FIG. 2 is a sectional view of the image sensor device of FIG. 1 taken onthe line II--II.

FIG. 3 is a sectional view of the image sensor device of FIG. 1 taken onthe line III--III.

FIG. 4 is a graph of the electric potential in the image sensor deviceaccording to the invention versus the depth, x, into the device.

FIGS. 5, 6 and 7 are cross-sectional views of successive stages in themanufacture of the image sensor device shown in FIGS. 1, 2 and 3.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The Figures are all schematic and are not drawn to scale. For the sakeof clarity, the dimensions in the direction of the thickness of thedevice are greatly exaggerated in comparison to the other dimensions.Semiconductor zones of the same conductivity type are cross-hatched inthe same direction. Corresponding parts are generally denoted by thesame reference numerals.

FIGS. 1 to 3 show an image sensor device comprising a semiconductorsubstrate core region 1 of a first conductivity type. In the example thesubstrate is n-type silicon. A surface 2 of the substrate is adjoined bya number of channel regions 7 of the first conductivity type. Channelregions 7 extend perpendicular to a system of electrodes 3, 4, 5 and 6present on the surface 2. During operation of the device, charge iscollected and transported in channel regions 7. Channel regions 7 areseparated by channel separation zones 8 of the second conductivity typeopposite to the first conductivity type. Separation zones 8 adjoin thesurface 2 and further adjoin an intermediate semiconductor zone 9 of thesecond conductivity type extending substantially parallel to the surface2. The electrodes 3, 4, 5 and 6 are insulated from the surface 2 by aninsulating layer 10 of, for example, silicon oxide.

During operation, as will be described hereinafter, voltages are appliedto the electrodes 3, 4, 5 and 6 to produce a pattern of potential wellsseparated by potential barriers in the channel regions 7. In thesepotential wells formed beneath the electrodes 4, 5 and 6, for examplecharge produced in the semiconductor materials 7 and 9 by incidentrafiation is collected for a given integration time. Thus, in a firstpart 11 of the image sensor device a charge image is formed whichcorresponds to the radiation image.

After the integration time, clock pulses are applied to the electrodes3, 4, 5 and 6 and to electrodes 12, 13, 14 and 15. As a result, thecharge packets are transported through the channel regions 7 and aretransferred, for example, to a storage register 16. The charge can thenbe read from this register 16 during the next integration time forfurther signal processing. The storage register 16 is covered by areflecting aluminium layer not shown so that the charge in this registercannot be modified by incident radiation.

During operation, voltages are applied between the electrodes 3, 4, 5and 6 and the substrate 1 to form a potential barrier at the area of thesemiconductor zone 9. Charge produced above this potential barrier willcontribute to the formation of the charge image. Charge produced beneaththis potential barrier will not contribute to the formation of thischarge image. Since long wavelength radiation can penetrate more deeplyinto the semiconductor material than short wavelength radiation, thespectral sensitivity of the image sensor devive can be varied by varyingthe position of the potential barrier.

According to the invention, in the image sensor device of FIGS. 1, 2 and3, the channel regions 7 have concentrations which exceed that of thesemiconductor zone 9, which in turn exceeds that of the substrate 1. Inthe example, the dopant concentration of the n-type channel regions 7 isabout 10¹⁶ atoms/cm³, the dopant concentration of the p-typesemiconductor zone 9 is about 3.10¹⁵ atoms/cm³ and the dopantconcentration of the n-type substrate 1 is about 5.10¹⁴ atoms/cm³. Thesemiconductor zone 9 further has a thickness which varies in a directionperpendicular to the channel regions 7, with minima at the centers ofthe channel regions 7. Due to these measures, the electric potential inthe image sensor device will vary in a direction perpendicular to thesurface 2 as shown in FIG. 4.

In FIG. 4, the potential V, at the center of each channel region 7, isindicated as a function of the distance X from the surface 2. Thesemiconductor zone 9 is at ground potential and the semiconductorsubstrate 1 is connected to a voltage of about -15 V. The curve 20 ofFIG. 4 represents the potential variation at the beginning of anintegration period. A potential well 21 is located in the channel region7 and a potential barrier 22 is located at the depth of thesemiconductor zone 9.

During the integration period, the potential in the device can vary asshown in curves 23 and 24 due to the collection of negative charge inthe channel region 7. When the situation indicated by the curve 24 isreached, only a small potential barrier 26 remains between the potentialwell 25 in the channel region 7 and the substrate 1. When a furtherquantity of charge is produced at this area, this charge can flow overthe small barrier 26 to the substrate 1.

Potential wells can be formed in the longitudinal direction of thechannel regions 7 by applying to the electrodes 3, for example, avoltage V₂ and by applying to the electrodes 4, 5 and 6 a voltage V₁.Beneath the electrodes 4, 5 and 6 the potential variations 20, 23, and24 respectively, are then obtained. Beneath the electrode 3, thepotential variation 27 is obtained. (In FIG. 4, the voltage drop acrossthe insulating layer 10 is indicated by dotted lines). Thus, any excesscharge in a potential well will flow away to the substrate 1 instead ofto adjacent potential wells in the channel region 7. This latterphenomenon, which is often designated as "blooming", can give rise tovery disturbing lines in a television picture if it is not avoided.

The "anti-blooming" described can be obtained with very practicalvoltages between the electrodes 3, 4, 5 and 6 and the substrate if,according to a preferred embodiment of the invention, the semiconductorzone 9 is interrupted at the centers of the channel regions 7 andexhibits slots 17 at this area where the n⁺ channel regions 7 extendthrough to substrate core region 1. (FIG. 3)

As described below, the image sensor device can be manufactured in asimple manner to produce channel regions 7 which are wider than channelseparation zones 8. The image sensor device according to the inventionthus has a comparatively high sensitivity.

FIGS. 5, 6 and 7 show successive stages in the manufacture of the imagesensor device shown in FIGS. 1 to 3. FIG. 5 shows the first stage ofmanufacture. Semiconductor substrate 1 of the first conductivity type,is in this case n-type silicon having an average dopant concentration ofabout 5.10¹⁴ atoms/cm³. Zones 33 of the second conductivity type, inthis case p-type, are formed by diffusion of impurities intosubstrate 1. The diffusion is via a mask 30 of, for example, siliconoxide with windows 32 at fixed center-to-center distances 31. Thesezones 33 have average dopant concentrations of about 3.10¹⁵ atoms/cm³.

Next, as shown in FIG. 6, channel regions 7 of the first conductivitytype, so in this case n-type, are provided via a second mask 34. Mast 34is, for example, also made of silicon oxide. Mask 34 has windows 35 atthe same relative center-to-center distances 31. By diffusion ofimpurities through mask 34, the structure shown in FIG. 7 is formed.

The second mask 34 is arranged so that the channel regions 7 are formedhalfway between the zones 33. The channel regions 7 are then separatedfrom each other by channel separation zones 8. Channel region 7 furtheradjoin the zone 9 extending substantially parallel to the surface 2.This zone 9 has a thickness which varies in a direction perpendicular tothe channel regions and has minima at the centers of the channel regions7.

The channel regions 7 have dopant concentrations, in this embodiment, ofabout 10¹⁶ atoms/cm³ on an average. These concentrations exceed that ofthe zone 9, which is about 3.10¹⁵ atoms/cm³ on an average. The dopantconcentration of zone 9, in turn, exceeds that of the substrate 1, whichis about 5.10¹⁴ atoms/cm³.

The widths of the channel regions 7 are equal to the widths of thewindows 35 in the second mask 34 plus the distance over which lateraldiffusion of the channel regions 7 occurs. This distance isapproximately equal to the depth of the channel regions 7 and amounts,for example, to 1/μm. A limiting factor is the minimum separation whichmust be maintained between two adjacent windows 35. If a mask can bemade in which this separation is 4/μm, the widths of the channel regionsbecome about 8/μm, and the widths of the channel separation zones 8become about 2/μm, when the center-to-center distance is 10/μm.

After an insulating layer 10 and systems of electrodes 3, 4, 5, 6 and12, 13, 14, 15 have been provided in a usual manner after removal of themask 34, the structure of FIGS. 1 to 3 is obtained. In FIG. 1, the slotregions 17 in zone 9 do not extend beneath the electrodes 12, 13, 14, 15of the storage register 16 because there is no danger of blooming here.However, there is no objection against providing slots 17 here too.

Preferably, the slots 17 have widths, a, in a direction perpendicular tothe channels 7, which exceed half the thickness, b, of the semiconductorzone 9, measured just beside the channel separation zones 8. (FIG. 3) Inthis case, at a substrate voltage of +15 V and a voltage at the channelseparation zones of O V, V₁ can be about O V and V₂ can be about -5 V.

In order to collect as much charge generated in the channel separationzones 8 as is possible, the channel separation zones 8 have widths, e,in a direction perpendicular to the channel regions, which are smallerthan four times the thicknesses, d, of the channel regions 7, measuredjust beside the channel separation zones 8. Thus, it is ensured that ina direction perpendicular to the channel regions 7, the potentialvariation causes charge produced in the channel separation zones 8 toflow to the adjacent channel zones 7 and not to the substrate 1 via thezone 9.

The slots 17 are obtained in a simple manner when the aforementioned twodiffusions are performed so that the zones 9 of the second conductivitytype do not contact each other, but are separated by regions 7 and 1 ofthe first conductivity type.

The invention is not limited to the embodiment described above, but manyvariations are possible without departing from the scope of theinvention. For example, the electrode system on the surface 2 may havelight windows and may comprise electrodes overlapping each other.Furthermore, the electrode system may be, instead of the 4-phase clocksystem shown, a 3-phase or 2-phase clock system. In the latter case, andin the case in which the electrode system has light windows, additionalsemiconductor zones of the second conductivity type may be formed in thechannel regions in order to obtain desired potentials in the channelregions.

Furthermore, the potential beneath the electrode 3 can vary as indicatedby curve 28 in FIG. 4, by applying a voltage V₃ to the electrode 3voltage V₃ is between V₁ and V₂. Thus, a potential well 29 is formed. Asa result of well 29 the sensitivity of the sensor will be improved sincecharge generated beneath electrode 3 now will flow to the channel regionbeneath electrodes 4, 5 and 6 instead of to the substrate 1. (Chargeflows to substrate 1 when the potential is as indicated by curve 27.)

What is claimed is:
 1. A method of manufacturing an image sensor device,said method comprising the steps of:providing a semiconductor substratehaving a conductivity of a first type, having a top surface, and havinga first dopant concentration; providing a first mask on the top surfaceof the substrate, said first mask having a number of windows extendingin a longitudutinal direction, said windows having a constantcenter-to-center spacing; diffusing impurities into the substrate viathe windows in the first mask to form zones of a second conductivitytype opposite the first conductivity type, said zones having a seconddopant concentration greater than the dopant concentration of thesubstrate and extending into the substrate a first depth, the depthvarying in a direction perpendicular to the longitudinal direction withthe depth having maxima beneath the center lines of the zones of thesecond conductivity type; removing the first mask; providing a secondmask on the top surface of the substrate, said second mask having anumber of windows extending in the longitudinal direction, said windowshaving a constant center-to-center spacing equal to the center-to-centerspacing of the windows in the first mask, the windows in the second maskbeing centered between the zones of the second conductivity type;diffusing impurities into the substrate via the windows in the secondmask to form a number of channel regions in the substrate, said channelregions adjoining the top surface of the substrate, said channel regionsextending in a longitudinal direction parallel to the top surface, saidchannel regions having a conductivity of a first type, said channelregions having a third dopant concentration which is greater than thedopant concentration of the zones of the second conductivity type, saidchannel regions extending into the substrate a second depth which isless than the depth of the zones of the second conductivity type;removing the second mask; and providing a system of electrodes above thetop surface of the substrate, said electrodes extending perpendicular tothe longitudinal direction and parallel to the top surface.
 2. A methodof manufacturing an image sensor device as claimed in claim 1,characterized in that in the diffusion steps the impurities are diffusedto depths chosen so that the zones of the second conductivity type areseparated by zones of the first conductivity type.